A first known type of pneumatic brake booster comprises an envelope equipped with a first and second shell crimped together in a sealed manner via their radially external ends. The shells are made of pressed steel sheet.
In addition, the booster is associated with a master cylinder mounted fixedly with respect to the booster by means of a flange formed integrally with the body of the master cylinder.
In a known way, the flange of the master cylinder is trapped between an external face of the booster envelope and a nut screwed onto a projecting threaded end of the booster envelope.
However, as it is desirable to secure the master cylinder to the booster prior to mounting in the engine compartment, it is necessary for this additional step to be simple and quick.
In addition, in order to give the envelope, which is subjected to high stresses because of the high variations in pressure, sufficient rigidity, the thickness of the sheet is relatively thick and increases the weight of the booster. Now, motor manufacturers are seeking to reduce the weight of each of the components of a motor vehicle in order for example to reduce its fuel consumption.
To remedy this problem, a second type of booster equipped with reinforcements connecting the first shell to the second along the axis of the booster so as to make it possible to reduce the thickness of the sheet used to make the shells while at the same time maintaining the rigidity of the envelope of the conventional booster is known.
However, the current tierods are complicated to fit and therefore increase the booster assembly time and therefore the cost thereof. Furthermore, the weight saving obtained can still be improved upon. The tierods also make it possible to attach the booster to the wall, known as the bulkhead, separating the engine compartment from the passenger compartment. A threaded longitudinal first end of the tierod passes through the bulkhead through an orifice made therein and is secured to this bulkhead by a nut mounted from the passenger compartment. The tierod also allows the master cylinder to be secured to the booster using a nut screwed onto the tierod.
In addition, when work is being done on the braking circuit, it may be necessary to replace the master cylinder. Now, the current mode of attachment means that the booster has also to be detached from the bulkhead.
U.S. Pat. No. 5,410,880 envisages a booster with tierod equipped with a first sleeve tube secured to the shell of the booster on the side on which the master cylinder is mounted, the sleeve tube being equipped on its external periphery with a screw thread collaborating with a nut trapping the flange of the master cylinder against said shell of the booster. The tierod is mounted in the sleeve tube and so to detach the master cylinder from the booster all that is required is for the nut to be unscrewed and there is no need to remove the tierod and therefore detach the booster from the bulkhead.
However, such a solution is expensive because of the need to secure a first sleeve tube in a sealed manner to a shell of the booster. In addition, the sleeve tube and the nut are both made of steel so as to be able to withstand the tensile stresses applied by the flange of the master cylinder. In consequence, the weight saving remains modest. In addition, the master cylinder is attached to the booster prior to mounting in the engine compartment by a screw-nut system which takes a long time to perform.